A high density interconnect (HDI) structure offers many advantages in the compact assembly of electronic systems. For example, a multi-chip electronic system (such as a microcomputer incorporating 30-50 chips) can be fully assembled and interconnected by a suitable HDI structure on a single substrate, to form a unitary package which is 2 inches long by 2 inches wide by 0.050 inches thick. Even more important, the interconnect structure can be disassembled from the substrate for repair or replacement of a faulty component and then reassembled without significant risk to the good components incorporated within the system. This is particularly important where many (e.g., 50) chips, each being very costly, may be incorporated in a single system on one substrate. This repairability feature is a substantial advance over prior connection systems in which reworking the system to replace damaged components was either impossible or involved substantial risk to the good components.
Briefly, in this high density interconnect structure, a ceramic substrate such as alumina which may be 25-100 mils thick and of appropriate size and strength for the overall system, is provided. This size is typically less than 2 inches square, but may be made larger or smaller. Once the position of the various chips has been specified, individual cavities or one large cavity having appropriate depth at the intended locations of differing chips, is prepared. This may be done by starting with a bare substrate having a uniform thickness and the desired size. Conventional, ultrasonic or laser milling may be used to form the cavities in which the various chips and other components will be positioned. For many systems where it is desired to place chips nearly edge-to-edge, a single large cavity is satisfactory. That large cavity may typically have a uniform depth where the semiconductor chips have a substantially uniform thickness. The cavity bottom may be made respectively deeper or shallower at a location where a particularly thick or thin component will be placed, so that the upper surface of the corresponding component is in substantially the same plane as the upper surface of the rest of the components and the portion of the substrate which surrounds the cavity. The bottom of the cavity is then provided with a thermoplastic adhesive layer, which may preferably be a polyetherimide resin (such as "ULTEM.RTM." 6000 resin, available from the General Electric Company, Fairfield, Conn.), or an adhesive composition described in U.S. Pat. No. 5,270,371, herein incorporated in its entirety by reference. The various components are then placed in their desired locations within the cavity and the entire structure is heated to remove solvent and thermoplastically bond the individual components to the substrate.
Thereafter, a film (which may be "KAPTON.RTM." polyimide, available from E. I. du Pont de Nemours Company, Wilmington, Del.), of a thickness of approximately 0.0005-0.003 inches (approx. 12.5-75 microns), is pre-treated by reactive ion etching (RIE) to promote adhesion. The substrate and chips must then be coated with "ULTEM.RTM." 1000 polyetherimide resin or another thermoplastic adhesive to adhere the "KAPTON.RTM." resin film when it is laminated across the top of the chips, any other components and the substrate. Thereafter, via holes are provided (preferably by laser drilling) through the "KAPTON.RTM." resin film, and "ULTEM.RTM." resin layers, at locations in alignment with the contact pads on the electronic components to which it is desired to make contact. A metallization multi-layer, with a first layer comprising titanium and a second layer comprising copper, is deposited over the "KAPTON.RTM." resin layer and extends into the via holes to make electrical contact to the contact pads disposed thereunder. This metallization layer may be patterned to form individual conductors during the deposition process or may be deposited as a continuous layer and then patterned using photoresist and etching. The photoresist is preferably exposed using a laser to provide an accurately aligned conductor pattern at the end of the process. Alternatively, exposure through a mask may be used.
Any additional dielectric layers for isolation between the first metallization layer and any subsequent metallization layers may be added by spinning on or spraying on a solvent solution of a desired dielectric adhesive material onto a thermosetting dielectric layer. Presently a siloxane polyimide/epoxy (SPIE) blend adhesive is used as an adhesive to bond additional layers of "KAPTON.RTM.". Since dielectric materials are used both in adhesive and in dielectric layers, there are special requirements placed on the system. In particular, in order for the final structure to be suitable over a wide temperature range, the dielectric layers (including the adhesives) must have high melting points and high thermal stability. Any candidate layer must provide good adhesion to the underlying dielectric and metallization and to overlying dielectric layer, and should also be inherently laser ablatable or should be rendered laser ablatable in accordance with U.S. Pat. No. 5,169,678 entitled, "Laser Ablatable Polymer dielectrics and Methods." Any misposition of the individual electronic components and their contact pads is compensated for by an adaptive laser lithography system which is the subject of some of the patents and applications listed hereinafter.
The process of designing an interconnection pattern for interconnecting all of the chips and components of an electronic system on a single high density interconnect substrate normally takes somewhere between one day and five weeks. Once an interconnect structure has been defined, assembly of the system on the substrate and the overlay structure is built-up on top of the chips and substrate, one layer at a time. Typically, the entire process can be finished in less than one day, as described in U.S. Pat. No. 5,214,655, entitled "Integrated Circuit Packaging Configuration for Rapid Customized Design and Unique test Capability" by C. W. Eichelberger, et al., herein incorporated in its entirety by reference. Consequently, this high density interconnect structure not only results in a substantially lighter weight and more compact package for an electronic system, but enables a prototype of the system to be fabricated and tested in a much shorter time than is required with other packaging techniques.
This high density interconnect structure, methods of fabricating it and tools for fabricating it are disclosed in U.S. Pat. No. 4,783,695, entitled "Multichip Integrated Circuit Packaging Configuration and Method" by C. W. Eichelberger, et al.; U.S. Pat. No. 5,127,998, entitled "Area-Selective Metallization Process" by H. S. Cole et al.; U.S. Pat. No. 5,127,844, entitled "Area-Selective Metallization Process" by H. S. Cole, et al.; U.S. Pat. No. 5,169,678, entitled "Locally Orientation Specific Routing System" by T. R. Haller, et al.; U.S. Pat. No. 5,108,825, entitled "An Epoxy/Polyimide Copolymer Blend Dielectric and Layered Circuits Incorporating It" by C. W. Eichelberger, et al.; and U.S. patent application Ser. No. 08/239,785, "High Density Interconnect Structures Incorporating An Improved Dielectric Material and Method of Fabrication", by H. S. Cole, et al. Each of these Patents and Patent Applications, including the references contained therein, is hereby incorporated herein in its entirety by reference.
As stated above, conventional high density interconnect (HDI) processes often use cavities formed into a substrate base for the placement of chips so that the top surfaces of the chips are essentially planar with the surface of the substrate. The substrate is generally a ceramic or a composite structure. The conventional HDI technique for fabricating cavities in the substrate is to mechanically machine or mill out the cavity material with a computer-controlled diamond tooled bit. This time consuming process does not always provide the desired chip cavity depth and can result in cracks which render the substrate unusable.
Chips are placed into these milled out cavities on multiple drops of die attach adhesive for mechanical, thermal, and electrical mounting. Chips placed with this process can be displaced during further processing because there are non-uniform surface tension forces at the chip-to-die attach adhesive interface. This displacement reduces precision in chip location, requiring further processing steps to adapt each electrical interconnection to the chip misalignment. Also, the presence of moats surrounding the chips in conventional substrates may cause thinning of the adhesive of the polymer film at the chip perimeters and sagging of the polymer film over the moats, thus adding difficulty in placing vias and patterning interconnects close to the chip wells. Additionally, mismatches between the coefficients of thermal expansion of ceramic substrates and polymer overlays sometimes induce stress at the adhesive layer, thus tending to promote separation of the polymer film from the substrate.
U.S. Pat. No. 5,353,498, entitled "Method for Fabricating an Integrated Circuit Module", to Fillion et al. discloses a method of fabricating an HDI substrate by molding plastic around chips placed on a film, thus eliminating the milling process and providing a planar surface without moats between chips and the substrate. Briefly, the technique includes applying an insulative base sheet over a base. At least one chip having contact pads is placed face down on the base sheet. A mold form is positioned around a desired perimeter and surrounds at least one chip. Substrate molding material is added and then hardened within the mold form. Then the mold form and base are removed, the substrate is inverted, and the chips are interconnected. When the molding material surrounds, and is in direct contact with the chips, stresses build up due to the difference in the coefficients of thermal expansion of the silicon of the chips and the polymer matrix of the molding material.
To reduce this stress, one embodiment of U.S. Pat. No. 5,353,498 places a thin sheet of polymer over the backside of the chips prior to the addition of the substrate molding material. This leaves air-moats between the chips and reduces somewhat the stress buildup; however, as stated above, the presence of these moats may cause thinning of the adhesive of the polymer film at the chip perimeters and sagging of the polymer film over the moats. Also, thermal plugs, useful for removing heat from modules, cannot be encapsulated by this thin polymer sheet due to the extreme difference in thickness between a chip with, and without, a thermal plug.
Consequently, it would be desirable to have a plastic molding process in which the mold form becomes an integral part of the substrate that protects the substrate from exposure to chemicals and, additionally provides a mechanism to reduce stresses built up within the module.